Some catalysts have two or more types of catalytic activity. This bifunctionality is believed to result from the presence, on the surface of the catalyst, of different types of catalytic activity sites. There are situations where it is desirable to suppress one type of catalytic activity while retaining the other. For example, in heteroatom removal catalysts, i.e., those that remove sulfur and other heteroatoms from a hydrocarbon feed, it may be desirable to suppress the catalyst's hydrogenation activity while retaining its heteroatom removal activity. Suppressing a catalyst's hydrogenation activity would be desirable when, for example, the catalyst is to be used for desulfurizing hydrocarbon streams containing desirable olefins, diolefins and aromatic unsaturates, such as naphthas for motor gasoline (mogas), diesel fractions, coker gas oil, and the like. Suppressing a catalyst's hydrogenation activity would also decrease the consumption of valuable hydrogen during the heteroatom removal process.
As an example specific to mogas, the primary mogas blend stocks are derived from FCC naphthas which, in addition to unwanted sulfur compounds, contain olefins which provide octane. The naphtha is reacted with hydrogen in the presence of a sulfided hydrodesulfurization catalyst, which removes the sulfur as hydrogen sulfide. At the same time, at least a portion of the olefins desirable for octane are saturated. Under relatively severe heteroatom removal conditions, a portion of the aromatics may also be saturated. Some naphtha desulfurization processes use catalysts that have been at least partially deactivated by coke formation or by the use of inhibitors, to decrease the olefinic saturation that accompanies desulfurization. However, the partial deactivation also substantially reduces the sulfur removal activity of the catalyst, which is undesirable. Other naphtha processes use hydrodesulfurization catalysts modified with metal compounds, which selectively and permanently poison hydrogenation sites. The hydrogenation activity of these catalysts cannot be restored, even with regeneration. Such processes are disclosed, for example, in U.S. Pat. Nos. 5,286,373; 5,525,211; 5,423,975; 5,985,136 and 6,231,754.
A process improvement would result if the hydrogenation activity of a catalyst, having activity for heteroatom removal and hydrocarbon saturation (hydrogenation), could be selectively suppressed, while preserving the heteroatom removal activity. A further improvement would result if this selective suppression could be achieved with the catalyst on-line in a reactor. Such on-line, selective suppression could be accomplished without taking the reactor off-line, removing the catalyst, treating the catalyst, recharging the reactor, and then restarting the process.